Refactor subnets mgmt in net-vpc-factory (#3715)

Refactor subnets mgmt in net-vpc-factory

This commit removes the ability to define subnets inline within the VPC `.config.yaml` across `net-vpc-factory` and enforces file-based subnet definitions using the `subnets/` subdirectory pattern.

Key changes include:
- **Module Updates:** Removed standard and non-standard inline subnet arguments (`subnets`, `subnets_private_nat`, `subnets_proxy_only`, `subnets_psc`) from the factory module's internal `vpcs` instantiation.
- **Schema & Docs:** Renamed `vpc.schema.json` to `vpc-factory.schema.json`, stripping inline subnet definitions from the JSON schema, and updated the module `README.md` to reflect the new file-based only approach.
- **Stage 2 Networking Configs:** Refactored datasets to migrate inline `subnets_proxy_only` definitions into standalone files like `subnets/prod-proxy.yaml`. Added a new exported output `subnet_ips` for downstream usage.

<!--
**Breaking Changes**

```upgrade-note
`fast/stages/2-networking`: Proxy-only subnets have been renamed for consistency - unfortunately this results in a nasty create-before-destroy which needs to be handled manually (i.e. delete the existing proxy-only-subnet first, either manually or with a targeted apply, then apply again)
```
-->

tests/fast/stages/s2_networking/nva.yaml

@@ -31,7 +31,7 @@ counts:
   google_compute_router_nat: 1
   google_compute_router_peer: 2
   google_compute_shared_vpc_host_project: 3
-  google_compute_subnetwork: 4
+  google_compute_subnetwork: 5
   google_compute_vpn_tunnel: 2
   google_dns_managed_zone: 5
   google_dns_record_set: 3
@@ -44,5 +44,5 @@ counts:
   google_storage_bucket_object: 2
   modules: 36
   random_id: 3
-  resources: 199
+  resources: 200
   terraform_data: 2

tests/fast/stages/s2_networking/vpns.yaml

@@ -26,7 +26,7 @@ counts:
   google_compute_router_nat: 1
   google_compute_router_peer: 10
   google_compute_shared_vpc_host_project: 3
-  google_compute_subnetwork: 3
+  google_compute_subnetwork: 4
   google_compute_vpn_tunnel: 10
   google_dns_managed_zone: 5
   google_dns_record_set: 3
@@ -39,5 +39,5 @@ counts:
   google_storage_bucket_object: 2
   modules: 30
   random_id: 15
-  resources: 214
+  resources: 215
   terraform_data: 2